The subject invention relates to a microporous organic polymeric membrane and the method for manufacturing the same. By "microporous membrane" it is meant a fluid permeable sheet or film having pores with a pore size of from 0.02 to 15 microns and having a thickness of less than 0.1 inches.
Microporous membranes have great utility both as filtration media and as permselective barriers which retain particles or liquids while allowing the passage of gases and vapors. As barriers to bacteria, they are well known for their use in sterile filtration of liquids and gases in the medical, pharmaceutical and electronics industry. In other applications, the microporous membrane is utilized as a sterile hydrophobic vent, allowing the passage of vapor but preventing the passage of an aqueous solution.
There exists a number of additional areas in which the introduction of a microporous barrier would be useful but as to this date, no practical article exists. Modern medical techniques to reduce infection introduced in the operating room have more recently come to rely upon the use of operating room garments and surgical drapes fabricated from disposable nonwoven fabrics whose fibers have been made hydrophobic. These materials are in effect depth type filters, whose open spaces are far larger than bacteria, but whose density is such that the bacteria has a high probability of encountering and sticking to such a fiber. Even the best of such fibers do not retain more than 92 to 95% of the airborne bacteria. The use of "dense" coated fabric, on the other hand, is not possible because the garment or drape must breathe; that is, allow transport of air and water vapor in order to avoid hyperthermia in the patient and provide comfort to the operating theater staff. The use of a microporous membrane (hydrophobic or hydrophylic) with a pore size appropriate to act as a bacterial barrier clearly suggests itself to such an application.
Other applications which require both the breathable aspect and the hydrophobicity include disposable sheets to protect bedding, breathable diaper exteriors and other like applications. Microporous membranes have not been commercially applied to these uses for reasons of cost associated with slow rates of production, and because they have not usually had the correct combination of barrier properties and mechanical flexibility and softness. Moreover, most such polymer films are mechanically flimsy and their practical use is only extended by bonding them to microporous fabrics or paper supports, thereby forming a laminate structure.
Methods exist for the preparation of such fabric or paper supported laminates. Where the microporous membrane can be formed as a free, stand alone film element, the laminate may be formed by direct dot gluing, heat embossing or the provision of an intermediary layer which is melted to join the layers by locking into the pores of both materials. All of these techniques have the disadvantage of "blinding" surface pores and reducing flow efficiency. Alternatively, under some circumstances the microporous membrane may be formed directly on the support material. A laminate of this type can only be formed provided that the polymer solution, from which the membrane is created, is of sufficient viscosity. Furthermore, the support material must be of sufficient density and not impaired either by the solvent system or the aqueous washing baths and drying ovens used in the process.
U.S. Pat. No. 4,466,931 assigned to the assignee of the instant invention discloses a method whereby a microporous membrane is prepared by the exposure of ultraviolet radiation or electron beam radiation of a solution of acrylic oligomers and/or monomers in a solvent or mixture of solvents which is a nonsolvent for the polymer formed as a result of the exposure. The class of monomers and oligomers most noted for this characteristic of undergoing rapid polymerization under electron beam or ultraviolet radiation are: the addition of polymerizable unsaturated organic compounds having a double bond between two carbon atoms, at least one of which has also bonded thereto a carboxyl, carboxylate ester or amino functionality; the epoxies and other cyclic ethers; and thiolenes.
When ultraviolet radiation of sufficient intensity in the 200 to 400 nm range is present, and in the presence of photoinitiator molecules which capture ultraviolet light and promote the polymerization reaction, or by the rapid injection of electrons from an electron beam, the process of polymerization and simultaneous phase separation can be made to take place with great rapidity. Consequently, production speeds greater than 100 feet per minute can be achieved. With an appropriate electron beam, line speeds in excess of 300 feet per minute are possible. The crosslinked microporous film produced by either technique must still be washed free of the original solvent, which remains in the pores after polymerization, and subsequently dried.
Due to the very high rate of manufacture compared to conventionally prepared microporous membranes, there is great advantage to the preparation of an ultraviolet or electron beam polymerized membrane as a laminated layer bonded to a fabric or paper support material. However, due to the low viscosity of the solutions normally used (10 to 50 cps), it is very difficult to utilize conventional methods of surface coating. Such techniques will not support the thin liquid layer unless the support material is quite dense.
Additional problems are presented with support materials which do not have a homogeneous superstructure. Defects or openings in the superstructure are non-uniform. Unless the support is extremely dense, such supports are difficult to coat without the formation of defects, such as holes, in the coated layer. In addition, the line speed must be sufficiently fast such that the residence time of the coating solution on the support, until completion of the polymerization, is shorter than the time for it (the solution) to wick into the support matrix.
Another problem relates to the preparation of hydrophobic membranes from ultraviolet or electron beam curable materials. In U.S. Pat. No. 4,466,931, all of the oligomers tested yielded intrinsically hydrophilic membranes. Moreover, conventional post-preparation treatments to render the membrane hydrophobic, such as dipping the membrane in solutions containing silicone or perfluorocarbon additives do not yield satisfactory results. Neither did the addition of vinyl terminated silicones or long chain hydrocarbon acrylate esters to the coating solution.
Additional problems include identification of the specific chemical structure of an oligomer appropriate to utilize for several of the applications of interest, which require flexibility and toughness. At the same time the material must also have sufficient mechanical strength so that the forces due to the large internal surface area do not cause the collapse of the porous structure. Similarly, the washing solvent which is used to wash out the curing solvent must not attack or soften the microporous structure and cause the film to collapse.